The present invention especially relates to the processes described in application Ser. Nos. 703,138; 703,139; 703,208 and said applications filed Dec. 29, 1977 and Jan. 4, 1978, since it improves the initial recovery of product acid when starting-up any of these processes.
In these applications, phosphate rock and sulfuric acid are reacted under conditions which result in the formation of solid calcium sulfate (either hemihydrate or gypsum) and phosphoric acid. A multi-vessel reaction system is used in which the reaction slurry undergoes intra- and inter-vessel circulation. Preferably, the inter-vessel circulation is through a draft tube. This results in excellent dispersion of reactants and minimization of temperature and concentration gradients throughout the slurry. In the hemihydrate process, the solution portion of the slurry in a first vessel (the "dissolver") is preferably maintained at a negative sulfate concentration (i.e. excess dissolved Ca.sup.+2) and the solution in a second vessel (the "crystallizer") is preferably maintained at a positive sulfate ion concentration. Also preferred is that the second vessel be maintained at a reduced pressure (e.g. to provide evaporative cooling). Better filtration rates can thus be obtained due to the favorable shape, dominant size and size distribution (especially, low fines content) of the calcium sulfate crystals. Most preferred is that a crystal modifier (e.g. a sulfonic acid, a sulfonic acid salt, tall oil, fatty acids or esterified tall oil, fatty acids) be present in the crystallizer. A two vessel hemihydrate process is described in U.S. Pat. No. 3,418,077 of Robinson, issued Dec. 24, 1968. Also relevant is the article by J. G. Getsinger, "IV Hemihydrate by the Foam Process" in Phosphoric Acid, Part 1, edited by A. V. Slack, at pages 369-382, Marcel Dekker, Inc., New York (1968), which shows a one vessel process.
U.S. Pat. No. 3,522,003 to Lopker shows CO.sub.2 removal by venting but from a separate pipe from the feed inlet (see also U.S. Pat. No. 3,522,004).
The present invention is directed to the manufacture of phosphoric acid by the wet process wherein phosphate rock is dissolved or reacted in phosphoric acid to produce a solution or slurry of monocalcium phosphate. The hemihydrate, or as it is sometimes called, the semihydrate, process is employed to produce wet phosphoric acid from phosphate rock and sulfuric acid. Phosphate rock and phosphoric acid (which can contain H.sub.2 SO.sub.4 and is preferably pre-mixed in a separate slurry tank) are added to a first reaction vessel of set of reactors, in parallel or series, (the "dissolver") which contains a first slurry comprising calcium sulfate hemihydrate, monocalcium phosphate, and phosphoric acid. The "soluble" or "excess" sulfate content (i.e., the excess or deficiency of sulfate ions over calcium ions) of the first slurry in the first reaction vessel can be maintained (e.g. by addition of sulfuric acid) at a concentration of about +0.7% to about -4% or even -8% (more preferred 0.0 to -6%), as determined for example, by the well-known gravimetric analysis. It is more preferred to be at a negative sulfate (e.g. excess Ca.sup.+2). The sulfuric acid in the first (dissolver) vessel is usually contained in "recycle" phosphoric acid from a filtration step and/or sulfuric acid contained in a side stream or recycle slurry from the second reaction vessel. If two dissolver vessels are in series, it is preferred that slurry from the second vessel (which has an excess of Ca.sup.+2, therefore, no free sulfuric acid) is used as the dissolution medium in the first dissolver vessel (e.g., the first vessel would be at about -6% SO.sub.4 and the second vessel at about -4%, caused by addition of sulfuric acid only to the second vessel). This would greatly reduce the "lattice bound" P.sub.2 O.sub.5 loss. This method of operation is the invention of James Moore, John Ellis and Gary Beer and will be the subject of a later filed application.
Sulfuric acid is added to the second reaction vessel which contains a second slurry comprising calcium sulfate hemihydrate, monocalcium phosphate, sulfuric acid and phosphoric acid. The sulfuric acid reacts with the monocalcium phosphate and any residual, undissolved phosphate rock, producing calcium sulfate hemihydrate and phosphoric acid. The soluble or excess sulfate concentration of the second slurry is maintained at a positive value (about +0.7% to about +4.5%).
Sulfuric acid is added in amounts such that the sulfate content of the added acid and the sulfate content of the added rock is equivalent to about 90% to about 100% (more preferred 93-99.5%) of the stoichiometric amount of sulfate required to react with calcium added in the phosphate rock to form calcium sulfate hemihydrate.
As is well-known in the art, sulfate and/or sulfuric acid can be introduced as such or as a part of a phosphoric acid "recycle" (as from the filtrate from filtering to separate the hemihydrate).
In order to maintain the desired soluble sulfate concentration in the first reaction vessel and in the second reaction vessel, circulation between the two reaction vessels is initiated. A first portion of the first slurry from the first reaction vessel is circulated through a first conduit into the second reaction vessel, and a first portion of the second slurry from the second reaction vessel is circulated through a second conduit into the first reaction vessel. This circulation is continuous. In order to better disperse the added phosphate rock and the added sulfuric acid within the slurry of the first and the second reaction vessels respectively and to better disperse the incoming slurry with the slurry present in the given reaction vessel, a second portion of the first slurry and a second portion of the second slurry is circulated within the first and second reaction vessels, respectively, preferably each through its own draft tube preferably at a rate equal to at least 50% of the volume of the slurry in a given reaction vessel per minute. This inter- and intra- vessel circulation disperses the reactants within the slurry in the respective reaction vessels. A third portion of the second slurry is removed from the reaction system so as to separate the liquid and solid components from the said slurry.
Although such patents as U.S. Pat. No. 3,939,248 to Caldwell and U.S. Pat. No. 2,968,544 to Zietz show the uses of draft tubes in phosphoric acid processes, these patents are not concerned with the problems of feed tube gas blockage or foaming or with hemihydrate processes, which are well-known to involve different deposition or scaling behavior than do gypsum processes.
Although the present process has been described as involving two reaction vessels, it should be understood that additional vessels (including reaction vessels) can be useful in the process and, for example, the dissolver can comprise two or more vessels in series or in parallel.
Especially preferred is the use of an additional vessel as a slurry tank, into which phosphoric acid (which can be a filtrate, usually containing in the range of 0.5-3.5% sulfuric acid) and phosphate rock are contacted to form a slurry which is transported (as by an overflow pipe) to the dissolver vessel. In such a slurry tank, as in the dissolver, it is frequently useful to use venting and/or to add a defoaming agent. A preferred defoaming agent comprises tall oil, tall oil fatty acids, or lower alkyl esters of tall oil fatty acids, or mixtures of such tall oil acids and esters, because such tall oil additives can also function (alone or with added sulfonics) as crystal modifiers. The preferred esters are the methyl and ethyl (as exemplified by the esters found in the commercial product marketed under the tradename AZ-10-A).